This invention relates to a system for protecting an electric vehicle which is driven by controlling the vehicle driving induction motor with a variable voltage/variable frequency inverter.
FIG. 1 shows one example of the main circuit of an electric vehicle whose induction motor is driven by an inverter.
The vehicle includes a pantograph 1 receiving electrical power for the vehicle. The power is switched by a switch 2 and is filtered by a filter reactor 3 and a filter capacitor 4. The filtered power is inverted by an inverter 5 that includes a thyristor 5a, particularly a gate turn-on thyristor, the inverted power is delivered to an induction motor 6 to which is attached a speed detector 7. The speed detector is optional.
The operation of the circuit shown in FIG. 1 will be described. In the electric vehicle as described above, the torque T provided by the induction motor 6 can be approximately represented by the following expression (1): ##EQU1## where V is the inverter output voltage, f.sub.INV is the inverter frequency, f.sub.S is the slip frequency, R.sub.2 is the motor secondary resistance, and k is a proportionality constant.
As is clear from equation (1), if V/f.sub.INV is maintained constant, then the torque T changes only with the slip frequency f.sub.S. In the power running operation, the inverter frequency f.sub.INV can be expressed as the sum of two frequencies. EQU f.sub.INV =f.sub.M +f.sub.S ( 2)
where f.sub.M is the motor rotational frequency, On the other hand, in the electric braking operation, the inverter frequency f.sub.INV is given by EQU f.sub.INV =f.sub.M -f.sub.S ( 3)
The inverter frequency f.sub.INV is determined as described above. As a result, the value V/f.sub.INV is made constant by adjusting the motor voltage V, and the slip frequency f.sub.S is controlled, so that the produced torque T has a predetermined value.
A problem arises because of the motor secondary resistance R.sub.2 changes with temperature. In order to overcome this problem and obtain a predetermined torque, the thermal change of the motor secondary resistance R.sub.2 is compensated and the variation of (f.sub.S /R.sub.2) in equation (1) is prevented by correcting the slip frequency f.sub.S so that (f.sub.S /R.sub.2) and consequently the motor current remain constant and a predetermined torque T is obtained. This has been described in detail, for instance, in the specification of Japanese Patent Application No. 181687/1983.
FIG. 2 is a diagram outlining the arrangement of a conventional electric vehicle control system of this type, adapted to drive a plurality of parallel-connected induction motors. As shown in FIG. 2, speed or rotational frequency detectors 7a and 7b are provided for the output shafts of induction motors 6a and 6b, respectively. In FIG. 2, a current detector 8 detects the current supplied to the two motors 6a and 6b and may be a current transformer. The elements 1 through 5 designate the same elements as those in FIG. 1. In the case of FIG. 2, a system is employed in which only the current instruction value is controlled as a value corresponding to the number of induction motors. The system is employed as a general drive system for controlling an electric vehicle. The term "current instruction value" is intended to mean the control target value for an induction motor which is determined according to predetermined data such as, for instance, the load of the electric vehicle to be controlled. The current instruction value is unequivocally determined from the torque value, the motor characteristic and other parameters which are required to obtain a predetermined speed control characteristic for the electric vehicle.
However, two problems arise with the described system. First, the induction motors are different in motor rotational frequency from one another because the wheels associated with the induction motors are likely to be somewhat different in diameter from one another. Accordingly, speed detectors are provided for all the motors and are selectively used for control. That is, in the power running operation, first the inverter frequency f.sub.INV is determined by adding the slip frequency f.sub.S to the minimum of the detected motor speeds f.sub.M (rpm). If the maximum of the motor speeds detected is larger than the inverter frequency f.sub.INV thus determined, in spite of the power running operation, the inverter frequency f.sub.INV is made higher than the maximum motor rotational frequency in order to avoid the occurrence of the event that the slip frequency becomes negative and the motors are placed in the electric braking mode. Similarly, in the electric braking operation, the situation should be avoided in which the motors are placed in the power running mode.
A second problem arises in the case where induction motors are parallel-connected as described above. A readhesion phenomenon is observed in which, when slippage (loss of traction) of the wheels coupled to the induction motors occurs (hereinafter referred to as "skidding"), the skidding frequency of the skidding shaft is decreased so that the motor current and the torque are decreased. However, in the case where the slip frequency f.sub.S is corrected so that the motor current is held constant as was described before, control is so made that the slip frequency f.sub.S is increased thereby to prevent the decrease of the motor current. Therefore, the other shafts may also be caused to skid. In order to prevent this difficulty, a readhesion control operation is carried out in which, when the skidding occurs, the skidding is detected by utilizing the variation rate df.sub.M /dt of the output signal of the rotational frequency detector provided at the shaft of each induction motor. Here f.sub.M is the rotational frequency, and t is the time. The result of the readhesion control is to decrease the current instruction value.
FIG. 3 shows another example of the conventional electric vehicle control system which is to drive not only parallel-connected induction motors on its own vehicle (hereinafter referred to as "a first vehicle" or master vehicle) but also parallel-connected induction motors on a second vehicle called a slave vehicle. One reason for the simultaneous control is to efficiently utilize the inverter 5. In FIG. 3, parts corresponding functionally to those already described with reference to FIGS. 1 and 2 are therefore designated by the same reference numerals. Further in FIG. 3, rotational frequency detectors 7c and 7d are provided for induction motors 6c and 6d, respectively, on the slave vehicle. In the conventional system, it is necessary to transmit the output signals of the rotational frequency detectors 7c and 7d provided respectively for the induction motors 6c and 6d on the slave vehicle to the master vehicle.
The inverter may be more efficiently utilized by increasing the number of parallel-connected induction motors. In this case, it is also necessary to simultaneously control the induction motors not only on the master vehicle with the controller but also on the additional slave vehicle. The vehicle with the controller will be called the master vehicle and the vehicle without the controller that is controlled by the master vehicle will be called the slave. Transmission between vehicles may be difficult if not impossible because of noise and the power source arrangement. Thus there also may be great difficulty when vehicles having greatly different wheel diameter are connected together.